1. Field of the Invention
The present invention relates generally to the protection of stainless steel from stress corrosion cracking and, more particularly, to the protection of flow-conducting components, such as pipe and fittings, which are constructed from stainless from chloride stress corrosion cracking when the stainless steel components are used in hot geothermal brine service in which the hot brine is likely to spill or leak onto the exterior of the components.
2. Background Discussion
Subterranean reservoirs of aqueous geothermal fluids--steam, hot water and hot brine--exist in many regions of the world. Such geothermal fluid reservoirs, many of which contain vast amounts of thermal energy, are most common where the earth's near-surface thermal gradient of the earth is abnormally high, as is evidenced by unusually great volcanic, fumarole, and/or geyser activity. As an example, significant geothermal fluid sources are found along the Pacific Ocean Rim--a region long known for its high level of volcanic activity.
Aqueous geothermal fluids have, in some inhabited regions, been used for centuries for the therapeutic treatment of physical disorders. In these and/or in some other inhabited regions, such as Iceland and the Paris Basin of France, geothermal fluids have also long been used as heat sources for industrial processes and for heating dwellings and other buildings. Moreover, in some places, such as Italy and Northern California, geothermal steam has been successfully used for a number of years to generate commercially significant amounts of electric power. In the late 1970s, for example, about 2 percent of all the electric power used in the State of California was produced by geothermal steam at The Geysers in Northern California, and presently enough electric power is generated at The Geysers to satisfy the combined electricity needs of the cities of San Francisco and Oakland, Calif. More recently, moderate amounts of electric power have been generated, notably in the Imperial Valley of Southern California near the Salton Sea, by geothermal brine, which is much more difficult to use than geothermal steam.
Such factors as the steep increases, in the early 1970s, in the cost of petroleum products and natural gas and projected future shortages and high costs of such resources have led to the recently increased interest in further developing the use of geothermal fluids as alternative, and generally self-renewing, electric power plant "fuels." Much of this effort has been and is being directed toward developing more economically practical systems and processes for using geothermal brine to generate electric power because, although more difficult than geothermal steam to use, there are many more good sources of geothermal brine than there are good sources of geothermal steam.
General processes by which geothermal brine can be used to generate electric power have, of course, been known for some time. Geothermal brine, having a wellhead temperature of over about 400.degree. F. and a wellhead pressure of over about 400 p.s.i.g., can, for example, be flashed to a reduced pressure to convert some of the water or brine into steam. Steam produced in this manner is then used in generally conventional steam turbine-type power generators to generate electricity. On the other hand, cooler, less pressurized, geothermal brine can be used in closed-loop, binary fluid systems in which a low-boiling point, secondary liquid is vaporized by the hot brine. The vapor produced from the secondary liquid is then used in a gas turbine-type power generator to generate electricity, the vapor being recondensed and reused. In both such cases, the "used" geothermal brine is most commonly reinjected into the ground to replenish the aquifer from which the liquid was produced and to prevent ground subsidence. Reinjection of geothermal brine is also often important to avoid problems typically associated with the disposal of the large amounts of saline and usually highly contaminated liquid involved.
In spite of such general processes for using geothermal brine for producing electric power being known, difficult and costly problems are commonly encountered with the actual use of the heavily contaminated, saline, and corrosive brines. Moreover, these problems are frequently so costly to overcome that the production of reasonable amounts of electric power at competitive rates by the use of geothermal brines has often been extremely difficult to achieve in many locations.
As mentioned above, many of these serious problems associated with the production and use of geothermal brines for the generating of electric power can be attributed to the unusually complex chemical composition and extremely corrosive nature of many geothermal brines. At aquifer temperatures and pressures--which are often well in excess of 400.degree. F. and 400 p.s.i.g.--aqueous geothermal liquids leach large amounts of salts, minerals, and elements from the aquifer formations, the geothermal liquids (brines) presumably being in chemical equilibrium with their producing formations.
Thus, although their compositions may vary considerably from location to location, geothermal brines typically contain very high levels of dissolved salts and silica, and appreciable amounts of dissolved metals and such non-condensable gases as hydrogen sulfide, ammonia, and carbon dioxide. Geothermal brines are usually acidic, with typical wellhead pH's of between about 5 and about 5.5. As a combined result of their composition and high natural temperature, geothermal brines can be some of the most corrosive liquids known.
Due to this extremely corrosive nature of many geothermal brines, brine-conducting pipe, fittings, and such other equipment as vessels, valves, and pumps, must often be constructed of expensive, normally corrosion-resistant metal alloys, such as special titanium alloys, nickel-based alloys, and/or various types of stainless steel alloys. The need for these costly alloys in geothermal brine handling facilities understandably results in high capital costs which must be passed on to the electric power consumers in the form of higher electricity rates. Since the cost of corrosion-resistant alloys is ordinarily directly related to how corrosion resistant the alloy is, it is, therefore, the general practice to select for any particular geothermal brine application that alloy (or those alloys) which is just sufficiently corrosion resistant for the intended use.
Corrosion problems associated with the production and use of geothermal brines are aggravated by the need for very large, continuous flows of brine to generate even relatively moderate amounts of electric power. As an illustration, the production of only about 10 megawatts of electric power (in a typical pilot plant operation) typically requires in excess of about a million pounds per hour of high temperature and high pressure geothermal brine. A more practical-sized geothermal power plant having about a 50 megawatt capacity typically requires between about 4 and 5 million pounds per hour of high energy geothermal brine. Accordingly, even moderate-capacity geothermal brine power plants ordinarily require several very costly brine production wells (each of which may be several thousand feet deep and cost on the order of a million dollars, exclusive of the cost of the production pipe which may be another million dollars). Also required are extensive amounts of large diameter, corrosion-resistant pipe, fittings, pumps, and valves, as well as at least several huge brine flashing and clarifying vessels, filters and so forth, and one or more brine injection wells, just for the production, steam-extraction, treating, and disposing of the huge flows of geothermal brine needed to produce steam for the associated electric power generating facility.
As above-mentioned, the selection of alloys for use in geothermal brine power plant service is typically based upon obtaining the degree of performance required at the lowest cost. Since the corrosiveness of geothermal brine is, to a great extent, directly related to brine temperature, the usual practice is to select alloys having the highest corrosion resistance (and, hence, usually the highest cost) for use in the highest temperature, downhole regions of brine production and handling systems. Especially corrosion-resistant (and expensive) alloys--such as high nickel-content alloys or special titanium alloys--are, therefore, now typically used or specified for down-hole brine production pipe to assure survival for the intended service life of the facility (which may, for example, be about 30 years). Nickel-based alloys may also be selected for use in furthest upstream regions of the above-ground brine handling system in which the brine remains at about wellhead temperature. Less corrosion resistant (and, hence, less expensive) alloys have been tested and/or commerically used in various services from the wellhead to the cooler, downstream portion of the brine handling facility, such as downstream of flashing stages in which brine temperature is substantially reduced as a result of the steam flashing process.
Representative of such less corrosion resistant alloys are the chrome duplex stainless steel alloys--which have less than about 15 percent nickel content and which are, for example, substantially less costly than most nickel-based or titanium alloys. Such alloys have been used in many applications from the wellhead to the cooler operational stages. On the other hand, austenitic stainless steels, which also contain less than 15 weight percent nickel and are substantially less costly than nickel based alloys, have less corrosion resistance and are therefore usually only used at temperatures below about 250.degree. F. As a result, these types of stainless steels are being used and/or are being designed for use in downstream, above-ground, brine-conducting piping, valves, fittings, vessels, and so forth, in some facilities for producing and handling corrosive geothermal brines.